This invention pertains to an autoclave being used for the pressure digestion of sample material for chemical analysis. It has a closeable vessel to contain the sample, which during operation is exposed to elevated temperatures and pressures. The vessel consists of a PTFE beaker and lid, which have sealing surfaces in direct contact with each other. The necessary pressure of the lid on the beaker is achieved by an extended clamping device which is remote from the seal joint between beaker and lid. This clamping device presses on the metallic lid support disc at only one point. This clamping device, lid/lid support disc, beaker/beaker support tube, and the free-standing lid/beaker seal act as a simple and effective excess-pressure release mechanism. Pressure digestion autoclaves have been known in diverse forms for a long time. To be mentioned are: W. Wahler (1964) "Mechanische und chemische Aufbereitung von Mineralien and Gesteinen fur geochemische Spurenanalysen" Neues Jahrb. Miner. Abh. 101, 109-126; L. Kotz et al. (1972) "Aufschluss biologischer Matrices fur die Bestimmung sehr niedriger Spurenelementgehalte bei begrenzter Einwaage mit Salpetersaure unter Druck in einem Teflongefass" Z. Anal. Chem. 260, 207-209; M. Stoepler and Backhaus (1978) "Pretreatment Studies with Biological and Environmental Materials" Fres. Z. Anal. Chem. 291, 116-120. In each case a closeable vessel of inert material (e.g. Teflon) is used. The sample under investigation is put into the beaker, and e.g., mineral acids are added after which the beaker is closed with the lid. The vessel is then heated whereupon the sample decomposes, forming a solution which can be further investigated. Since extremely high pressures can develop during decomposition of the sample, the vessel must be enclosed with a pressure safe container. To increase safety an excess-pressure release mechanism may be included.
The main disadvantage of these autoclaves lies in the fact, that due to the high internal pressures a loss of the produced gases is inevitable which, after contacting the metal container may re-enter into the vessel during the cooling phase leading to a contamination of the sample with those elements from which the container is made. Any determination of these elements will then be unreliable.
A further disadvantage is that the pressure safe (metal) container for the vessel also includes a complicated pressure safe closure mechanism which makes the whole unit large, heavy, and difficult to handle. Absolute certainty against undesirable ruptures of such units cannot be achieved, since extremely high pressure peaks can be produced through the reaction of the added acids with the sample of unknown composition. Pressure release valves can help to prevent explosive ruptures, but they require constant maintenance to ensure proper functioning. Furthermore their durability is rather limited by the corrosive vapors to which they may be exposed.